1. Field of the Invention
This invention is related in general to the field of electric energy detection systems. In particular, the invention consists of a device used to detect an energized equipment-grounding terminal in an electrical receptacle.
2. Description of the Prior Art
In a common wall receptacle outlet, an ungrounded circuit conductor, usually black, is customarily connected to a supply side of an electrical distribution system, and a grounded circuit conductor, usually white, is connected to a neutral side of the electrical distribution system. The grounded conductor provides a reference voltage for the receptacle outlet and is referred to as the “neutral”. The electric potential of the ungrounded conductor, relative to the neutral conductor, is defined as the line voltage, and the conductor is referred to as the “line” or “hot” conductor.
Modern electrical distribution systems typically include a third wire, a normally non-current-carrying equipment grounding conductor. This conductor provides a low impedance path for ground-fault currents necessary to activate over-current devices and also equalizes the potential between exposed metal surfaces. This equipment grounding conductor, for purposes of this disclosure, will be referred to as the “ground”, and the other two, normally current-carrying conductors, will be referred to as the “hot” and “neutral”.
Modern 125 volt receptacles typically have three terminals that correspond to the hot, neutral, and ground. These terminals are marked and color-coded, and only the corresponding conductors are supposed to be connected to the terminals Circuit detection devices (“testers”) are commonly used to determine whether the terminals of an electrical receptacle have been properly connected to the electrical distribution system. These testers are typically small plug-in devices that are inserted into the receptacle and provide an indication of the electric potential between the hot, neutral, and ground wires.
A typical tester may have up to three lamps for indicating electric potential between conductors. A first lamp, “Lamp 1”, is connected between the hot and neutral terminals of the receptacle. If the lamp illuminates, an electric potential exists between the two terminals. A second lamp, “Lamp 2”, is placed between the hot and ground terminals to indicate whether an electric potential exists between these two terminals as well. A third lamp, “Lamp 3”, is typically inserted between the neutral and ground terminals to serve the same function.
Using this type of tester, a normal condition is indicated by an illumination of the first and second lamps only, showing that an electric potential exists between the hot and neutral terminals and between the hot and ground terminals, but any electric potential between the neutral and ground terminals is less than that necessary to illuminate the third lamp. Lamps may be selected to indicate various levels of electric potential between the conductors. However, a typical lamp that is illuminated is intended to represent approximately 120 volts alternating current between conductors and a non-illuminated lamp is intended to represent little or no electric potential between conductors.
A normal condition exists when Lamp 1 and Lamp 2 only are lit. If Lamps 1 and 3 are lit, the Hot and Neutral are reversed. If Lamps 2 and 3 are lit, the Hot and Ground are reversed. If only Lamp 2 is lit, there is an open Neutral. If only Lamp 1 is lit, there is an open Ground. If no lamp is lit, there is an open Hot.
A typical tester is capable of detecting the mis-wiring conditions indicated above. However, there have been instances resulting in fatalities wherein the ground wire is not connected to a receptacle, the hot and neutral wires have been reversed, and the neutral and ground terminals have been electrically connected together, either with a jumper or wire. This condition is not typically detectable by today's circuit testers and is likely to occur when a modern three-terminal receptacle is installed in a two-wire electrical distribution system having only hot and neutral conductors. If the hot and neutral conductors are interchanged somewhere within the distribution system, or if they are connected to the wrong terminals at the receptacle, the neutral terminal is energized at line potential and the line terminal is grounded at the service entrance.
Under these conditions, if a connection is made, either intentionally or unintentionally, between the neutral and ground terminals, all exposed metal on the load side of the receptacle will become energized at line potential. With this scenario, all UL listed 3-light circuit testers will indicate a safe condition when in fact there is a serious shock hazard present. There is at least one known documented electric shock fatality traceable to this situation.
Modern electric equipment is often designed to take advantage of an electrical distribution system's ground conductor to prevent accidental injury or electrocution. For example, a modern hand-held drill insulates the hot conductor from exposed metal parts such as the frame or case of the drill. However, if a short should develop between the hot conductor and the frame of the drill, the user is protected because the frame of the drill is customarily connected directly to the ground conductor, allowing current to discharge through the ground wire rather than through the user. If the erroneous wire condition indicated above were to occur, the frame of the drill would not be connected to the electrical distribution system's ground, but rather would be energized. Simply touching the frame may create a circuit between the distribution system's energized supply side and the user, resulting in a severe electric shock that may injure or even kill the user. However, a traditional tester is incapable of detecting this condition.
If a traditional tester is plugged into a receptacle wherein the ground terminal is electrically connected to the neutral terminal and both are connected to the distribution system's hot wire while the hot terminal is connected to the distribution system's neutral wire, then lamps one and two would be illuminated while lamp three would be dark. In this scenario, lamp one would be placed between the now energized neutral terminal and the hot terminal sitting at reference voltage and would light up. Lamp two would be between the now energized ground terminal and the hot terminal sitting at reference voltage and would also light up.
Because both the ground terminal and the neutral terminal are energized, no significant electric potential exists between them, and therefore, lamp three would not be illuminated. The result is that a typical tester would indicate a normal wiring condition of the receptacle when, in fact, a potentially deadly hazard exits. The typical receptacle outlet circuit tester cannot indicate the presence of supply voltage on the ground terminal because the tester has no reference to earth ground. Accordingly, it would be advantageous to have a circuit tester device that can detect this anomalous condition.